Allow then_do to take an async function if the mock is async

[mcous]

The then_do method is a useful advanced method for some tests, especially when you're not in a position to change the API of a dependency at the time you're writing the test.

However, the then_do method only accepts a synchronous callable, even if the mock is an asynchronous mock. I've run into a few cases where this limitation was frustrating and I didn't have an immediately obvious path for better code structure within the constraints I needed to follow.

• If a mock is async, it should accept an async callable as its then_do side-effect
• Passing an async function to a sync mock should raise an exception